A large bottle is a good format to keep the wine in the cellar, it's exceptional and beautiful to look at but it has a price: the price of the work of the craftsman who bottled it.
What you need to know is that large bottles are produced by hand. Filling, corking and labeling are manual operations because the scarcity of these particular bottles does not justify the mechanization of these operations.
To the price of the wine which can be valued based on the volume, you must add expensive manual operations, bottles and packaging that are more expensive due to the size.
For example an empty magnum bottle of 1.5 liter will cost around 4 francs, a 3 liters double magnum will cost 12 francs and a 6 liters mathusalem will cost 68 francs. The progression is exponential. As for the corks, they will cost from 1 franc to 3 francs a piece.
It is therefore normal that prices increase with the size of the bottle. A large format bottle of wine becomes a rare object for exceptional moments; it makes a perfect gift and contributes to give a particular luster to an event.